double check that the form elements in the page are all configured correctly with a value attribute, even if the value is blank:
<input type="text" name="first_name" value="" >
I have seen some older versions of IE have issues passing form data with fields that did not include the value attribute.
make sure your client is actually clicking the forms submit button with the mouse rather than hitting the enter key to submit the form.
have them try the form with Fire fox instead of IE to see if the problem persists.